Coal gasification processes have one common problem in the injection of coal solids into the high pressure reactor; or the removal of spent solids from the reactor to atmosphere or some form of transportation system. Although the operating principle of a lock hopper system is relatively simple, the operation of the valves associated with such a system is complicated and difficult. The difficulties arise from the requirement of the valves of handling the highly erosive char on discharge side of the coal gasification process, and coal on the feed side of the system. This difficulty exists whether the material being handled is dry or in slurry form. The difficulty becomes more pronounced when the valves are also required to seal against high pressures and/or very hot gas. It is not uncommon in process systems to find pressures of at least 1600 psi and higher, and temperatures which are above 600.degree. F. Reliability of the valves for the severe service and for long-time intervals without failure adds to the complication of difficulties.
At a recent conference sponsored by the Department of Energy (DOE) and the Morgantown Energy Research Center in conjunction with the Valve Manufacturer's Association, the conferees were informed that valves have been tested for the aforementioned type of service and that all present problems and are not entirely adequate for the type of severe service outlined. However, this could be due to the fact that the valves tested might be off-the-shelf type and not conceived for the operation desired.
The lack of a market for valves required for the severe service would generally discourage manufacturers expending monies in doing R&D work. The need for a severe service valve in coal gasification processes and other types of processes which are suitable for handling a three-phase mixture of solids entrained in water and gas is more than evident; it is an urgent requirement.
It is herein proposed to utilize slide gate valves between a surge hopper and a lock hopper and between the lock hopper and atmosphere. Standard slide gate valves normally depend on rigid metal-to-metal contact between the sliding gate and its static seat located in the valve housing, to shut-off against the medium of materials or fluids (solid particles and gas under pressure in this application) passing through the valve. In such valves, clearances must be provided between gate and seat, even if all surfaces are machined to close tolerances, in order to allow the former to slide freely in the latter for opening and closing the valve. It is these very clearances that prevent the valve from being completely gas-tight in the closed position, when a gas pressure differential is applied across the valve. The solid particles only serve to aggravate the situation by getting trapped in between gate and seat, thereby preventing complete closure between their machined surfaces, and in fact cause these surfaces to become scored when the gate is operated to open or close the valve. Attempts to remove the trapped solid particles from the site by means of streams of pressurized gas (hereafter referred to as gas-pressure-purging) cannot be guaranteed to remove all the particles entirely from the site.
The above-described two basic problems exist also with other types (categories) of valves, such as ball valves, butterfly valves and swing-check valves. Again, complete closure between rigid-moving, male-member surfaces and their mating static rigid-seat surfaces is not possible, to provide a completely gas-tight shut-off against a gas pressure differential. Even when one of the aforementioned contact surfaces is not rigid, for example, when seats are made of teflon or other semi-rigid materials, any trapped solid particles remaining (and some will remain even after gas-pressure-purging) can become embedded in the teflon seat and will score both the seat as well as the moving male member when the valve is operated to open or close, due to the close machined tolerances (or clearances) maintained between the two parts.
The novel valve of the present invention includes an inflatable seal element located in the valve housing seat. There is also a gas-pressure-purge arrangement which purges the seat area of abrasive solid particles.
The inflatable seal element can be inflated by means of pressurized gas, to grow in size and close up clearances between gate and seat, thereby establishing a gas-tight seal between the two parts, at will. So this can be done, for example, after the valve has been operated to close (slide gate driven into the valve housing) and a gas-pressure-tight seal is desired. Prior to this event, the gas-pressure-purge arrangement is activated to blast-off solid particles from the gate surfaces where these make contact with the inflatable seal element. It is not mandatory that each and every solid particle be removed from the gate contact surfaces by means of the gas-pressure-purge arrangement. Only a reasonable amount of cleanliness on the gate contact surfaces is desirable. The inflatable seal element is capable of sealing gas-tight even when some solid particles become trapped and embedded on the inflatable seal surface. The purpose of the gas-pressure-purge arrangement is to keep the contact surfaces reasonably free of solid particles only to improve the life of the inflatable seal element.
Furthermore, the inflatable seal element can be deflated to shrink in size, thereby providing increased clearance between gate and seat, when desired. So this is done just prior to the moment when the gate is to be operated (driven out of the housing) to open the valve. No scoring can occur on gate or seat on account of the increased clearance.